My 2p worth.
I'd never recommend anyone buy a used PowerTap. Once they're out of warranty you're looking at around £425 to replace the torque tube. It's impossible to know just how long a torque tube will last, but anything beyond a few years seems to be quite unusual and a couple of years quite normal, especially with the SL+ hub which is poorly sealed and leaks water. My PowerTap Pro+ was trouble free for 2.5 years and then it just died. I decided not to pay the ridiculous £425 for repair and bought a Power2max instead (for about £750).
The other issue with PowerTaps is the restriction on wheel choice. I ended up swapping rims so I'd build the hub into a carbon clincher for summer which I'd use for training and racing and then an alloy rim for the winter. That worked I suppose, but it's not ideal.
My P2M is the SRAM S900 gxp version. The right crank is a bit tricky to find since its the older version, but they do come up on
eBay fairly often. I recently bought a spare for £65 which isn't bad for a really nice carbon crankset. Swapping between bikes is a 2 minute job - it really is that easy with gxp bottom brackets. So the claimed advantage of a PowerTap being easier to swap between bikes isn't really true.
As for data, I've had no reason to doubt the data from either the PowerTap or P2M. Hub based power is much more jumpy and there are more erroneous 1s spikes, but anything beyond a few seconds average is ok. The zero offset on my P2M is stable and as far as I can tell the temperature compensation update does it's job. Mine came with the temperature compensation stuff so I don't know what it'd be like without it. I've had no reason to doubt the data from my P2M even when I go straight out from hot-cold. After around 15min the zero offset settles and varies little through the rest of the ride. One possible negative for the P2M is that you can't set the slope (like you can with Quarq and SRM) so if you calibrate it and find a different slope from the calibration certificate then you'd have to return it to P2M in Germany for them to sort it out. One big plus for P2M is that they offer a half price crash replacement.
I've never owned a Quarq, but from reading the Google Wattage list (where lot's of very smart people (and the occasional idiot) discuss power meters) there do seem to be a lot of warranty returns and quite a few unhappy customers who've had recurrent issues. It's always hard to know just what percentage of owners have issues, but I'd say there are more unhappy Quarq customers than SRM, P2M or PowerTap customers.
SRM still is the gold standard for power meters. Returning them once or twice a year for a battery change is maybe a pain, but that's not a reason to not buy one IMO. I'd have one if I had a spare couple of grand. There's a prototype SRM with usb recharging anyway so the battery change issue may disappear soon.
Keep an eye out for Brim Bros pedal system which is maybe going to be released this year (or maybe not). I don't think we'll ever see the Garmin Vector.
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